Certain laser processing applications perform processing of a regularly spaced pattern of target locations on a workpiece. For instance, certain solar cell processing applications involve drilling vias through the silicon wafer in a regularly spaced grid pattern. Customers for these applications seek very high processing throughput, on the order of several thousand vias per second.
The spacing of vias in these applications is fairly dense, on the order of 0.25-1 mm. The overall processed area is significant, typically 150×150 mm square wafers. Therefore, the laser processing system processes this area by rapidly drilling the tight-pitch vias. The accuracy of such systems is on the order of 3-20 μm. The drill time for each via depends on laser characteristics (wavelength, pulse frequency, pulse power, and pulse width), via diameter, and substrate material and thickness. The drill time is, however, typically on the order of 0.1-0.5 msec. Via diameters are typically on the order of 15-50 μm.
Typical conventional processing system approaches rely on galvanometer (galvo)-based positioning of the laser processing beam, either alone (using a very large galvo field), or optionally combined with a secondary movable stage (and thereby permitting a relatively small galvo field). More recently, a tertiary acousto-optic deflector (AOD) stage has been implemented. It is noted, however, that these primary, secondary, and tertiary beam steering approaches have certain limitations.
A first system architecture implementing galvo-based processing laser beam positioning uses a single large galvo field to cover the entire workpiece. This implementation uses either a very large scan lens or a post-lens scanning system. In either case, the galvo typically moves the processing beam at a constant velocity over the entire workpiece, and a controller fires a laser pulse at each via location without stopping the galvo. A relatively small number of pulses are used for each via, so several processing passes are made to fully drill each via. Accordingly, a regularly spaced pattern of target via locations improves processing times. This approach avoids the timing overhead and thermal effects of frequent galvo acceleration and deceleration, because galvo turnarounds take place only at the edges of the workpiece. The measure of the angular deflection range at which the laser spot (or simply, spot) at the workpiece surface is not too distorted by the free-space optics and stays in focus is typically between 1,000 and 10,000 spot widths (or simply, spots) across one axis of scanning.
If a very large scan lens is used to cover the entire workpiece field, the large lens is subject to accuracy degradation caused by optics heating that results from working with high-powered laser beams. The large lens also uses a large beam diameter to obtain the desired workpiece surface spot size. Such large beam diameters use large galvos, which in turn suffer from accuracy effects resulting from the lower thermal efficiency of moving large (high-inertia) mirrors with large (high-inertia) galvos.
If a post-lens scanning system is used to cover the entire workpiece field, the lens thermal accuracy effects are reduced. The processing system suffers, however, from the effects of non-telecentric beam delivery, which degrades the quality of the drilled vias. Moreover, reducing such telecentric errors can be achieved by maintaining a long focal length, which would entail using a large beam diameter to obtain the desired workpiece surface spot size. This leads to thermal accuracy issues similar to those described above because of the large galvos employed in such systems. If telecentric errors are not of significance, one can use a shorter FL lens and avoid the non-flat focus field problem by using a dynamic focus element. Disadvantages of this approach include cost; complexity; inaccuracy contribution by the focus element; cost of the focus element for very high-speed applications; and residual telecentric error.
A second system architecture is a compound positioning system, in which a small galvo field (typically about 20 mm square) is implemented in conjunction with a structural mechanism that moves a galvo head over the workpiece (either through an X-Y workpiece table, or by a cross-axis moveable optics configuration). As in the first system architecture, the galvo may scan over the vias at a constant velocity, pulsing the processing laser beam at each via, to avoid the overhead of stopping at each via location. As the galvo rapidly scans over its field, the galvo spends a significant amount of time accelerating and decelerating at the edges of the scan field because it is significantly smaller than the workpiece. This expenditure of time causes a significant reduction in throughput, and if high acceleration is used to reduce the turnaround time, thermal heating of the galvo degrades accuracy and places an upper limit on achievable acceleration. However, the second system architecture does have the advantage of higher accuracy (resulting from reduced lens distortion with the smaller scan lens), improved via quality (resulting from the smaller, lower-distortion scan lens, and the telecentric scan field), and potentially high beam positioning speed (resulting from small galvos and mirrors). Yet this approach may be infeasible because of the throughput limitation described above, depending on the number of laser pulses used to process each via.
AODs in a tertiary beam position stage have a bandwidth (about 1 MHz) that is close to three orders of magnitude larger than that of galvos (about 2.5 kHz). Therefore, AODs enable error correction for the galvos as well as very fast beam steering within their range of deflection (approximately 10 to 50 spots). But state of the art lasers (and even more so for experimental lasers currently being developed) provide increasingly higher powers at increasingly faster repetition rates (e.g., from ones of MHz to hundreds of MHz). Also, some lasers can readily further scale power by increasing repetition rates well beyond 1 MHz and can thereby reach maximum average powers at 1.6 MHz and above. These bandwidths exceed a tertiary beam positioning system's capability to fully spatially separate each pulse from its neighboring pulses. Fully separated pulses are used by many processes in the laser-micromachining domain because, if pulses partly overlap, the following two negative effects take place: local heat accumulation negates the beneficial effects of a-thermal ablation provided by ultrafast lasers, and there is pulse-plume interaction.